The present invention relates to a valve device for evacuation of a gaseous medium with a pressure above atmospheric pressure from a space, comprising an atmosphere-communicating valve housing with a valve opening to said space, a valve body which is biased towards the closed position in contact with a valve seat about the valve opening by spring elements and towards the open position by the pressure of the gaseous medium, and a control plunger disposed in a cylinder, said plunger, upon supply of a control pressure medium to a cylinder chamber on its one side, being loaded by a force which counteracts the spring force.
In commonly occurring valve devices of this type, the plunger is joined directly via a valve spindle to a valve element in the form of a valve disc. The spring means biases the plunger, which in turn biases the valve disc to its closed position. When control pressure medium is supplied, the plunger is displaced in a direction so that it moves, via the spindle, the valve disc from the valve seat. In a known design, the valve disc and the valve seat are formed and arranged so that the spring presses against the plunger, which in turn presses the disc against the seat, while the pressure of the medium in the space as well as the control pressure in the cylinder chamber of the plunger act on the plunger in the opening direction, i.e. in the opposite direction In another known design, the spring presses against the plunger, which pulls the disc toward the valve seat. In this case, the force exerted by the gaseous medium acts in said space in the opposite direction, i.e. in the closing direction of the valve.
In the first example, the control pressure will be inversely proportional to the pressure of the gaseous medium, since this pressure acts in the opening direction. An increase in the set value of the medium pressure is thus obtained by reducing the control pressure. A homostable system is thereby achieved, since a disturbance in the form of an increase in the medium pressure in the space, for example, will result in the valve opening more and reducing the medium pressure without having to adjust the control pressure. One disadvantage of this valve is, however, the lack of precision since the spring mounting force tolerance and the spring constant will be included in the equilibrium equation. The spring must be mounted with a force sufficient to absorb the maximum overpressure of the medium in the space. Since the medium pressure and the control pressure act in the same direction, the control pressure range will only be circa one half of the medium pressure range, thus further reducing the precision.
In the second example, the control function is reversed, with the valve working in the opposite direction, i.e. closing automatically when there is medium pressure m the space, but it must be opened with the control pressure. This system is, however, completely unstable, since a disturbance where the medium pressure in the space increases also increases the closing force of the valve, which must immediately be dosed by increasing the control pressure. Systems functioning in this way are very difficult to precision-control.
A general purpose of the present invention is to achieve a valve device of the type described by way of introduction, which is kept closed by a spring force without supplying control pressure medium up to a predetermined medium pressure and through which, by supplying control pressure medium with a pressure up to a predetermined interval a medium pressure can be maintained which is lower than said predetermined medium pressure. The construction of the valve shall be such that the control precision will be practically independent of spring tolerances and clamping forces, and the medium pressure increases with increasing control pressure within said interval.
An additional purpose of the present invention is to achieve a valve device of the type described by way of introduction, which is particularly suited for controlling the charge air pressure in an intake line from a turbo-compressor on a supercharged four-stroke internal combustion engine of turbo-compound type, which is provided with throttle valve means in the exhaust line between the engine exhaust outlet and its charge turbine.
This is achieved according to the invention by virtue of the fact that the plunger and the valve body are movable relative to each other and are biased in opposite directions by the control pressure medium, that first spring means bias, via the plunger, the valve body towards the closed position, that second spring means bias the valve body towards the open position, and that the spring forces of the first and second spring elements and the piston area and the area of the valve body exposed to the gaseous medium, are selected so that the first spring element, in the absence of control pressure medium, keeps the valve body in the closed position until a predetermined maximum pressure has been reached in said space, and that the valve body, upon supply of control pressure medium with a pressure within a predetermined pressure interval, opens the valve to maintain a gas medium pressure dependent on the control pressure within an interval, the upper limit of which is lower than said maximum pressure.
When using such a valve as a charge air pressure-regulating valve in a turbocharged combustion engine with exhaust braking function, the valve has a first function in engine drive and a second function in braking operation. During engine drive, the valve is held closed by the spring force up to a maximum permitted charge air pressure and then opens under the influence of the charge air pressure and thus limits the charge-air pressure. During engine braking, control compressed air is supplied first up to a pressure level balancing the closing force of the spring means. By then increasing the control pressure during engine braking from the balancing pressure and up to an upper predetermined control pressure level, the charge air pressure during engine braking can be varied within an interval which is optimal for engine braking.
By selecting the spring forces and said areas so that the pressure interval of the control pressure has an upper limit which several times higher than the maximum permitted charge pressure, there is achieved a high resolution and precision in the control of the valve.